1. Field of the Invention
The present invention relates generally to flight director and/or fully automatic flight control systems for helicopters and more particularly to such systems for providing effective closed loop vertical path control and airspeed control over the normal operating envelope of the helicopter.
2. Description of the Prior Art
Helicopter flight director and/or autopilot systems have been in service for many years and typical of such systems are those disclosed in such of the present Assignee's U.S. Patents as U.S. Pat. Nos. 2,845,623, 2,916,052, 3,916,688, 3,967,236 and 4,109,886 among others. However, it should be pointed out that while the system may be referred to herein as a "three-cue" system, the present invention is concerned primarily with the control of the longitudinal path of the helicopter and hence involves only the cyclic pitch control cue and the collective pitch control cue for commanding the pitch attitude of the helicopter and its vertical movement respectively. Thus, the roll or direction cue is not involved (except when cross controlled with the pitch cue in an entirely conventional manner).
In many prior art three cue systems, when a vertical control mode is engaged, such as glide slope, vertical speed, altitude hold, without engaging an airspeed mode, the path error generated by the flight director computer is supplied to the cyclic pitch cue (or pitch cyclic pitch actuator of the autopilot) and a pitch attitude is commanded in a manner to reduce the path error to zero. However, if the airspeed is not sufficient to produce the required path change, for example, an airspeed less than say 50-70 kts. depending upon the type of helicopter involved, then pitch control alone becomes sluggish and unstable and the additional control of collective pitch becomes essential. Furthermore, if in a cruise mode at higher airspeeds and precise closed loop control of airspeed is required along with tight vertical path control, it is again necessary to engage collective pitch control. In the latter case, mode control logic places airspeed control in the hands off pitch cyclic pitch control and path error in the hands off collective pitch control. However, while tight control of both airspeed and path are achieved, collective pitch will become overly active in the presence of gusts or turbulence. In the flight director mode, this obviously increases pilot work load. Also, since most helicopters employ constant rotor speed control, i.e, collective pitch movement results in a change in engine throttles, such a three-cue system is wasteful of fuel.
Thus, in practice, many helicopter operators prefer to use full three-cue flight director capability at low airspeeds but at higher cruise airspeeds, they prefer to disconnect this collective cue and revert to two cue (roll and pitch) operation where vertical path errors are corrected by pitch cyclic pitch only. Unfortunately, this practice with conventional three-cue flight director/autopilot systems requires the sacrifice of closed loop airspeed control. Moreover, most helicopter autopilots do not provide automatic control of collective pitch and hence pilot's workload is substantially increased when switching from cruise (two-cue) to low speed maneuvering (three-cue).